There are many fans of -Peter Pan- I've met who didn't like Scarlet at all. I mostly hear what you've said above: it's not the same. The story is melancholy; the characters are more brittle than the original; the Neverland isn't what it was in Barrie's book. But, from how I understand it, that was McCaughrean's intent.
-Peter Pan in Scarlet- is, at least for me, a rather moving book for a novel intended for children. I empathize with Ravello, that tragic character who longs for the past but is powerless to wish it back; a character who has experienced horrific trials and instead of running from them, faces them, broken, but defiant. Ravello listens out and listens in...much like me.
And Pan. Peter's metamorphosis and eventual fall into the abyss of despair shows us something Barrie really didn't: Peter's humanity. As much as many fans of the One-and-Only-Child probably won't like to hear this, I really do like how McCaughrean took Peter's immortality away. She gave Peter weakness, thus giving her plot depth; her characters life.
I don't believe this sequel is the Anti-Peter Pan so much as a wake up call. McCaughrean's Neverland is dangerous, even deadly. It's a gritty world much like our own, where all of our decisions and actions have consequences; where the test of courage is weighed heavily upon our shoulders; where we often look ahead to brighter times - the peaks of mountains - but sometimes find nothing but memories of a distant past, haunting, ever haunting, and despair. In -Peter Pan in Scarlet-, McCaughrean took Barrie's dream-world of a child's ideal reality and washed a bit of the real world into it. Yes the story may be grim and dark, the language choppy and clipped, but it has much to say about the world in which we live.
In regard to the epilogue, however, I must say I don't believe it was entirely necessary. McCaughrean must have tacked it on for some reason other than this, but I have a creeping suspicion she only put it there because the novel "needed" a clean, happy end. Without the epilogue, the story would have ended on a rather dark note, something that is simply unheard of in most children's literature. Though it does wrap the story up, it wasn't entirely necessary and is thus awkward.
All in all though, I really enjoyed the book for its gritty messages and dark themes. It contrasts with Barrie's original, but in a tasteful, welcome way (to me, at least). But, as always, to each his own!